Over the last two decades MR microscopy has evolved into a subset of MR Imaging with a wide range of applications, with its greatest benefit still the ability to image live tissue non-invasively. Still, the resolution is limited compared to other microscopies and until recently cellular and subcellular resolutions on mammalian tissue were not possible. Additionally, the cellular origins of MR signals in tissues are still unknown, and mathematical models attempting to elucidate this issue are the subject of great debate. Recently, using microsurface coils at high fields, we have obtained the first direct MR images of mammalian cells, and further, fiber tract maps at the cellular level with direct histological correlation. Still, these studies were of fixed tissue and the data took several hours to acquire. This proposal will demonstrate that the combination of smaller microsurface coils, higher magnetic fields and smaller, faster and stronger planar gradient coils can, conservatively, improve the SNR by an order of magnitude or more. Then, coupled with new microperfusion chambers, mammalian sub-cellular resolution MR microscopy of live mammalian tissue can be achieved in physiologically acceptable imaging times. Additionally new microvolume coils will be developed for accurate quantitative studies. Aims 1-5 will implement MR microscopy at successively higher magnetic field strengths (14.1, 17.6 and 21 Tesla) using new microsurface and volume microcoils, new planar microgradients and optimized sequences, and testing the system for stability and accuracy of quantitation. We will explore the utility of these developments primarily on neural tissue (single Aplysia neurons and rat brain slices, both on fixed tissue and then live perfused tissue) and similarly in cardiac tissue. When successful, a wide range of tissues will be possible to study. Through quantitation of intra and extracellular signals and how they change with physiological perturbations (for example, ischemia), we will be able to develop working realistic mathematical models of MR signals in tissues. Additionally, we will be able to accurately validate fiber tracking techniques at the cellular level. Thus, MR microscopy will provide a complementary microscopy technique for imaging live tissue at the sub-cellular level. Relevance: The development of the MR microscope capable of imaging live mammalian tissue at the sub-cellular level in physiologically acceptable imaging times will for the first time facilitate a quantitative understanding of the signal origins in MRI. This in turn will impact the sensitivity and specificity of MRI, improving its clinical potential. For example, a quantitative understanding of the signal changes in brain and cardiac ischemia may be able to resolve the difference between reversible and irreversible damage in stroke and heart attack, and have a major impact in improving the utility of MRI in a wide variety of tissues and diseases. PUBLIC HEALTH RELEVANCE: An MR microscope will be developed capable of obtaining cellular and sub-cellular resolution in live mammalian tissue in physiologically relevant acquisition times using new microcoils, microgradients and a micro-perfusion system at high magnetic fields. Quantitative studies will be undertaken on live brain and cardiac tissue. Consequently an understanding of the origins in MR signals will be developed, impacting on the sensitivity and specificity of clinical MRI.